1. Field of the Invention
The present invention relates to a forged piston formed by forging a metallic material and a method of making such a piston, and also relates to an internal combustion engine and a transportation apparatus including such a forged piston.
2. Description of the Related Art
Recently, a forged piston, formed by a forging process, has been adopted more and more often as a piston for an internal combustion engine. The forged piston has excellent mechanical strength and abrasion resistance at high temperatures. That is why if a forged piston is adopted, the output can be increased by increasing the explosion pressure and the weight can be decreased by reducing the size (or the thickness) of the piston skirt.
For example, Japanese Laid-Open Patent Publication No. 2000-179399 discloses a forged piston made of an aluminum alloy. FIGS. 14A and 14B are respectively a top view and a side view schematically illustrating the forged piston 510 disclosed in Japanese Laid-Open Patent Publication No. 2000-179399, supra.
As shown in FIG. 14A, this forged piston 510 has a piston head 510 with fiber flows f extending radially. That is to say, these fiber flows f are parallel to the radial direction. The fiber flows f are also called “metal flows” or “flow lines”, which are the traces of metal structure flows as is often seen in a forged product. Also, as shown in FIG. 14B, the piston skirt 505 has fiber flows f extending parallel to the sliding direction.
In the forged piston 510 disclosed in Japanese Laid-Open Patent Publication No. 2000-179399, however, the fiber flows f of the piston head 501 extend parallel to the radial direction as shown in FIG. 14A. That is why if the weight were further reduced to improve the performance of the internal combustion engine, the piston could have insufficient endurance strength with respect to the bending stress applied either parallel or perpendicularly to the piston pin under explosion pressure. The reason why the bending stress is applied in those directions will be described with reference to FIGS. 15A and 15B.
As shown in FIG. 15A, the explosion pressure itself, caused by combustion, is applied isotropically to the piston head 501. However, the piston 510 has a thickened portion 503 (which is called a “piston boss”) defining a bearing for the piston pin as shown in FIGS. 15A and 15B.
For that reason, in a cross section of the piston 510 as viewed on a plane that includes the center axis of the piston 510 and that is parallel to the piston pin (i.e., a cross section including both the thickened portion and the thinner portion), bending stress is produced so as to press down the center portion, which is thinner than the piston boss 503 as shown in FIGS. 16A and 16B.
When such bending stress is produced to depress the center portion on a cross section as viewed parallel to the piston pin, bending stress that depresses the center portion is also produced on a cross section as viewed perpendicularly to the piston pin as shown in FIGS. 17A and 17B.
The present inventors discovered and confirmed via experiments that, if the weight of the piston 510 were further reduced, the piston head 501 with those radially extending fiber flows f might have insufficient strength with respect to the bending stress applied in those directions.
The same statement also applies to the piston skirt 505 that often has its thickness reduced to make the piston 510 even more lightweight. Specifically, in that case, if the fiber flows f extend parallel to the sliding direction of the piston 510 as shown in FIG. 14B, the endurance strength of the piston 510 could also be insufficient to endure the fatigue caused by repetitively applied impacts.